1. Field of the Invention
This invention relates to a solder material used for connecting electronic components mounted on a wiring substrate.
2. Related Art
As a solder material used for connecting electronic components mounted on an wiring substrate, a tin-lead based eutectic solder is predominantly used.
Recently, the problem of environmental pollution by lead has been pointed out mainly in Europe and the United States and the possibility of placing a ban is being discussed. For coping with this tendency, research for reducing the consumption of lead in the solder material or development of a lead-free solder material have been desired.
The tin-lead solder, predominantly used nowadays, has an eutectic structure of 63 wt % of tin and 37 wt % of lead, with the eutectic temperature being 183xc2x0 C. This tin-lead solder has its wettability optimized by the proportion of lead and superior wettability can be realized at lower temperatures to give optimum soldering properties.
On the other hand, the lead-free solder material, now investigated, is based on tin and is of a composition composed of plural elements for realizing properties comparable with the tin-lead solder. Specifically, various additive elements are added to the binary eutectic alloy of the Snxe2x80x94Ag, Snxe2x80x94Zn, Snxe2x80x94In and Snxe2x80x94Bi system for controlling the properties.
However, these lead-free solder materials are inferior to the tin-lead solder in temperature characteristics, such as melting temperature, or in wettability, such that a further improvement has been desired.
In particular, acceptability of soldering depends on the wettability of the solder material. That is, if the solder material is less susceptible to wetting, it is in poor contact with the component to be connected. Conversely, if it is wetted excessively, solder bridging is induced between components which should not be connected to each other, thus producing shorting. The higher the mounting density of the wiring substrate, the more acute is the problem ascribable to wettability, such that it is not too much to say that the quality of electronic components in the surface mounting technique depends primarily on the wettability of the solder.
In light of the above, the lead-free solder material, thus far proposed, encounters difficulties in controlling its wettability, because lead is not used, such that optimum soldering properties are not realized.
It is therefore an object of the present invention to provide a lead-free solder material which is superior in wettability to realize an excellent soldering performance.
In addition, in one aspect, the present lead-free solder also does not include a Sb material and therefore further limits environmental pollution to the Earth.
In one aspect, the present invention provides a solder material wherein a Snxe2x80x94Znxe2x80x94Bi alloy containing 0.5 to 10 wt % of zinc and 0.5 to 8 wt % of bismuth, with the balance being substantially tin, is further alloyed with 0.005 to 0.5 wt % of germanium and 0.3 to 3 wt % of copper.
It is possible for the solder material to contain 0.3 to 1 wt % of copper.
In another aspect, the present invention provides a solder material wherein a Snxe2x80x94Bixe2x80x94Ag alloy containing 0.5 to 8 wt % of bismuth and 0.5 to 3 wt % of silver, with the balance being substantially tin, is further alloyed with 0.01 to 0.1 wt % of germanium.
In yet another aspect, the present invention provides a solder material wherein a Snxe2x80x94Znxe2x80x94In alloy containing 3 to 15 wt % of zinc and 3 to 10 wt % of indium, with the balance being substantially tin, is further alloyed with 0.01 to 0.3 wt % of germanium and 0.3 to 3 wt % of silver.
Since the solder material of the present invention is a Snxe2x80x94Znxe2x80x94Bi alloy added to with germanium and copper, a Snxe2x80x94Bixe2x80x94Ag alloy added to with germanium, a Snxe2x80x94Bixe2x80x94Ag alloy added to with germanium and copper or a Snxe2x80x94Znxe2x80x94In alloy added to with germanium and silver, the solder material exhibits superior mechanical properties and satisfactory wettability while having an optimum melting temperature as a solder. Thus, it is possible with the lead-free solder material to realize excellent soldering characteristics comparable with the tin-lead solder.
These and other features of the present invention will become clearer with reference to the following detailed description of the presently preferred embodiments and accompanying drawings.